1. Field of the Invention
The present invention relates to a digital television system carrying out modulation/demodulation by VSB (vestigial side band).
2. Background of the Related Art
An 8VSB transmission system for terrestrial wave digital broadcasting is taken as a U.S. standard system in 1995 is test-broadcasted from the latter half of the year 1998. Such a test broadcasting using a terrestrial wave digital broadcasting standard based on the U.S. standard system is being carried out in Korea as well.
In such a digital broadcasting system, a transmitted signal is received by a receiver through a terrestrial channel. In order to restore the signal in the receiver despite the noise caused by the channel, the signal is variously coded to be transmitted. In response to the various coding process, the receiver carries out the corresponding decoding process so as to restore the transmitted signal.
Lately, a broadcasting station tries to transfer such a digital broadcasting, which transfers mainly audio and video data, to which various additional data are attached. The additional data includes stock-market information, weather casting, program guide information, HTML, execution files and the like.
FIG. 3 illustrates a structural diagram of a Trellis Coded Modulation (TCM) encoder according to a related art.
Referring to FIG. 3, a TCM encoder includes a precoder 1 outputting a first output signal by precoding a first input bit d1 and an encoder outputting a third output signal by encoding a second input bit as a second output signal c1, where a reference numeral ‘3’ indicates a 8VSB mapper.
Meantime, the precoder 1 includes an adder 1a outputting the first output signal c2 by adding the first input bit d1 to a delayed signal and a memory 1b providing the adder 1a with the delayed signal attained by delaying an output signal of the adder 1a. 
The TCM encoder according to a related art may cause a fatal damage on the additional data when using it.
Different from general audio/video data in channel transmission, the additional data is vulnerable fatally to an influence of the channel noise. For example, the damaged additional data of which information itself is defected may inform a viewer with wrong information, while the damaged general audio/video data just result in light image/voice loss.
Specifically, when the additional data include numbers or an execution file, a minor data error causes a devastating result of failing the entire operation.